Magnetic field-effects in bipolar, almost hole-only and almost electron-only tris-(8-hydroxyquinoline) aluminum devices

We present magnetoconductivity and magnetoluminescence measurements in sandwich devices made from the $\pi$-conjugated molecule tris-(8-hydroxyquinoline) aluminum $\left({\text{Alq}}_3\right)$ and demonstrate effects of more than 25% and 50% magnitude, respectively. These effects are known to be caused by hyperfine coupling in pairs of paramagnetic species, and it is often assumed that these are electron-hole pairs. However, we show that the very large magnitude of the effect contradicts present knowledge of the electron-hole pair recombination processes in electroluminescent $\pi$-conjugated molecules and that the effect persists even in almost hole-only devices.